1. Field of the Invention
The invention relates to vehicle mounted satellite antennae. More particularly, the invention relates to a slotted waveguide planar vehicle mounted satellite antenna with simultaneous dual polarization states, which can employ a hybrid electronic and mechanic steering mechanism, and is operable while the vehicle is in motion.
2. Related Art
It has long been known how to mount a satellite antenna (dish) atop a vehicle for purposes of communicating with a geostationary or other type of satellite. The initial applications for mounting a satellite dish on a vehicle were military communication and remote television news broadcasting. Consequently, the first methods of mounting a satellite dish included a telescoping mast which was hingedly coupled to the vehicle. When the vehicle was in motion, the mast would be retracted and folded with the satellite dish lying end up on the roof or a side wall of the vehicle. The dish would be deployed only when the vehicle was stationary. Such a deployable vehicle mounted satellite dish is disclosed in U.S. Pat. No. 5,961,092 to Coffield. Until recently, no vehicle mounted satellite antennae were operable while the vehicle was in motion. The relatively large size of a conventional satellite dish antenna presents significant wind resistance if deployed on a vehicle in motion. This wind resistance adversely affects the operation of the vehicle and subjects the satellite dish to potential wind damage. Moreover, satellite dishes must be accurately aimed at a satellite within a relatively narrow aperture or “look window”. In order to operate a satellite dish mounted on a vehicle in motion, it would be necessary to constantly re-aim the dish in order to maintain communication with the satellite.
Recently, satellite antennae have been developed which may be deployed on a vehicle and operated while the vehicle is in motion. Such antennae are disclosed in U.S. Pat. No. 5,398,035 to Densmore et al., U.S. Pat. No. 5,982,333 to Stillinger, and U.S. Pat. No. 6,049,306 to Amarillas. These antenna systems generally include a satellite antenna of reduced size and a solenoid system for aiming the antenna. The solenoid system is coupled to a feedback system and/or vehicle motion detectors in order to automatically re-aim the antenna as the vehicle is in motion. In order to reduce aerodynamic drag and protect the antenna from wind damage, an aerodynamic radome is often used to cover the antenna.
Vehicle mounted satellite antennae which are operable while the vehicle is in motion, can provide one-way or two-way satellite communications. Some applications for such antennae include satellite television reception, telephony in remote locations where cellular telephone service is unavailable, and broadband data communications. The application of television reception may be advantageously applied in common carrier transportation such as long distance buses, in recreational vehicles including boats, and in the rear seats of family mini-vans. The application of remote telephony may be applied in the same situations as well as in various other governmental and commercial settings. The application of broadband data communication may also be applied in many personal, commercial, and governmental settings.
Broadband satellite communication, such as television reception or broadband data communication requires a high gain antenna with high cross-polarization isolation and low signal sidelobes. Satellite antenna gain is proportional to the aperture area of the reflector. Stationary satellite antennae typically utilize a circular parabolic reflector. Reflector type of satellite antennae designed for use on a moving vehicle is difficult to achieve low profile. In order to maintain gain, these low profile antenna are short but wide so that the overall aperture area is kept high. However, this design strategy only works to a point. When the width to height ratio exceeds a certain value such as 2, the efficiency of the antenna is adversely affected. The presently available vehicle mountable dish reflector type of satellite antennas, for commercial and personal use, are no shorter than approximately fifteen inches in height. A mobile satellite antenna produced by Audivox Corp. (MVSTS Satellite TV System) provides four circular Casegrain dish reflector antennas positioned along a horizontal axis perpendicular to the direction of antenna aiming. The signals received by the four dish reflectors are combined in phase to achieve aggregate antenna gain. Since the signal arriving at the phase centers of the four reflectors with the same propagation delay, no phase shifters are required for this mobile satellite antenna. The use of four reflector dishes allow the width to height ratio to be stretched further, while maintaining the antenna efficiency. The overall height of this antenna including radome is approximately 9.5 inches, considerably reduced from the single reflector type of dish antenna. Another mobile satellite antenna produced by Titan corporation (DBS-2400 Low Profile Ku-Band Antenna System) uses four hemisphere Luneberg lens antennas positioned on top of a ground plate along a horizontal axis perpendicular to the direction of the antenna aiming. The signals received by four Luneberg lens antennas are combined. The use of the ground plate to create an image of the hemisphere antenna reduces the height of the Luneberg lens by half, to approximately 5 inches (including radom). Another approach described in U.S. Pat. Nos. 6,657,589 and 6,653,981 to Wang et al., is a linear cylindrical Casegrain reflector antenna with line source. Such antenna profile is also limited to approximately 5 inches without elongating the antenna length prohibitively. A common drawback of the antennas described above is that two dimensional mechanic movement and control is required to aim the antenna toward satellite. This makes the mechanic design complicated and reduces the reliability of the antenna system. Another drawback of these types of antennas is that the height of the antenna is still too large for esthetically mounting on top of the roof of the commercial vehicles such as mini-van or SUV (Suburban Utility Vehicle). Further, the Lunberg lens antenna approach is heavy and expensive.
Another approach for implementing the mobile satellite antenna is to employ a phased array antenna having a large number of antenna elements. An antenna aiming in the azimuth and elevation directions is achieved by passing the received signal from each antenna element through a phase shifter. The phase shifter rotates the phases of the signals received from all antenna elements to a common phase before they are combined. While such antennas can be implemented with a very low profile, the large number of microwave processing elements such as amplifiers and phase shifters used in the electronic beam forming network results in high implementation cost, preventing mass volume commercial use. One of such antenna was published by V. Peshlov et al. of Sky Gate BG, IEEE 2003, Phased-array antenna conference.
U.S. Patent application Nos. 2003/0083063, 2003/0080907 and 2003/008098 describe an antenna mounted on a horizontal platform, which is rotatable to adjust the antenna beam in the azimuth direction driven by a motor, and is also capable of steering the antenna beam in the elevation direction through an electronic beam forming network.
Waveguide antennas are typically less than one wavelength in height and provide signal combining along the waveguide longitudinal axis. Many forms of waveguides can be used for microwave energy transmission. Rectangular waveguides have currents flowing on its interior wall and interrupting those currents by cutting through the waveguide wall can cause radiation into the exterior. It is well known, and used, that a radiating aperture is achieved when that aperture is approximately one-half free space wavelength long and one twentieth of a wavelength wide is cut through the broad wall of that waveguide. The aperture is widely described as a “slot” through the waveguide wall. Locating such a slot at various positions on the waveguide wall achieves varying degrees of excitation of microwave fields emanating from the slot. The microwave fields from the simple slot are characterized as being linearly polarized microwave fields.
Many applications for field radiating structures require that the radiated fields have the property of being circularly polarized. A widely used technique for producing a circular polarized radiating element is the cutting of a pair of slots through the broad wall of a rectangular waveguide. The two slots are typically caused to cross each other at ninety degrees to each other, and at the center of each slots length. Further, the crossed slot is normally placed on a line that is parallel to the waveguide axis and is a distance of approximately one quarter of the waveguide width away from the waveguide axis.
U.S. Pat. No. 3,503,073 to James Ajioka et al., and subsequently in IEEE Transaction On Antenna and Propagation, March 1974, describes using a dual polarized slot radiators in bifurcated waveguide arrays. The radiating element is a pair of crossed slots in the narrow wall of a bifurcated rectangular waveguide that couples even and odd modes. One linear polarization is excited by the even mode, and the orthogonal linear polarization is excited by the odd mode. Alternatively, one circular polarization can be excited through one of the pair of waveguides, whereas, the other circular polarization can be excited through another waveguide in the pair. The above-described antenna design approach has the drawback of unequal propagation velocities of the even and odd mode within the waveguide which causes the even and odd beam to point at different direction. In order to equalize the two group velocities, very narrow compensating slits within the waveguide wall are used, which reduces the waveguide bandwidth and significantly complicates the manufacturing complexity.
Another antenna described in IEEE Transaction of Vehicular Technology, January 1999 by K. Sakakibara et al., employs X-shaped slot located in the broadwall of a rectangular waveguide, approximately halfway between the center line and the narrow wall, to form a two-beam slotted leaky waveguide array. The broad side width of rectangular waveguide is approximately half the waveguide, and the cross slot center is offset from the center of the waveguide toward the sidewall by approximately 90 mil. The slot spacing along the waveguide is 0.874 inch. Such waveguide spacing can result in grating lobe when the beam is steered to different elevation angle. At higher elevation angle, the grating lobe becomes comparable in strength to the main lobe, thereby reduces the antenna gain. A right-hand circular polarization can be achieved by feeding the waveguide from one end, whereas a left hand circular polarization can be achieved by feeding the waveguide from the opposite end. One disadvantage of this antenna is that the beam direction of the right-hand polarization antenna is different than the beam direction of the left-hand polarization antenna. As the user switches from one polarization to the other polarization, the antenna rotates in azimuth direction in order to refocus the antenna toward the satellite, resulting in temporary disruption of signal reception. The antenna described above is designed for a fixed elevation beam angle.
U.S. Pat. No. 6,028,562 to Michael et al. describes a planar array of waveguide slot radiators of parallel waveguides which couples the electromagnetic signal from alternating +45 degree and −45 degree radiating slots interfaced on top of the waveguide to the slots on the broadwall of the waveguides via cavities which serve as impedance matching network. In a corresponding U.S. Pat. No. 6,127,985 to Michael et al., a similar slotted waveguide structure is employed. A T-shaped ridge waveguide is employed to realize closely spaced waveguide slot radiator to provide simultaneous dual polarization and suppression of grating lobes. The Michael patents have the disadvantage of complicated manufacturing processing. In addition, the patents use a rear-fed waveguide combining structure, which is not intended for electronic beam steering.
Conventional systems have focused the antenna beam toward the satellite while vehicle is moving using a mechanic dithering approach. In this approach, the antenna is rotated in both azimuth and elevation by a small angle, such as a fraction of the antenna beamwidth, to slightly off-point the antenna beam in the left, right, up, and down directions. The mechanic dithering involves controlling a motor to move the antenna platform. This approach has the shortcoming of a slow response and inaccuracies in the mechanic movement require the use of motion sensors (such as gyro, accelerometer, or compass) to aiding the tracking thereby resulting in significant signal degradation. Electronic dithering is faster, but still subject to the similar problems of slow response. The motion sensors are expensive.
It is desirable to provide a vehicle mounted satellite antenna which has simpler mechanical control and more reliable design.